drbd: Change how the initial packets are called

The first packets exchanged when a connection is established are
referred to as P_HAND_SHAKE_S and P_HAND_SHAKE_M in the code, followed
by P_HAND_SHAKE packets.  To avoid confusion between these two unrelated
things, call the initial packets P_INITIAL_DATA and P_INITIAL_META.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>